In many processing and communication systems, two balanced voltage signals are utilized to produce a single output signal. The balanced voltage signals are transmitted to a differential receiver or amplifier whose output is proportional to the difference between the two input voltage signals. Typically, twisted pairs of signal wires have been utilized to transmit the balanced differential signals. In this manner, if noise is introduced into the balanced signals, the noise affects the two signals of the wires of the pairs in proportional amounts and the difference remains the same so that the output signal is unchanged.
Heretofore, twisted pairs of insulated conductor wires have been woven in flat cable form to provide balanced transmission lines. By twisting associated wires together in pairs their impedance is sought to be controlled. However, the impedance of such a cable is difficult to control. The impedance varies with the spacing of the wires in each twisted pair which is a function of the uniformity and thickness of the insulation surrounding the wire. In the extruding of these wires it is difficult to keep the wire centered in the insulation so that variances occur in the thickness of the insulation and wire-to-wire spacing in the twisted pairs. Furthermore, the impedance is affected by the number of twists per unit length which is difficult to control. The wires also vary in stiffness along their length which causes the wires not to lay as close to each other in the stiff areas further varying the wire-to-wire spacing and affecting impedance values.
Laminated twisted pairs of wire are also utilized in balanced line transmission cables such as shown in U.S. Pat. No. 4,359,597. Twisted sections alternate with straight sections in the cable whereby the cable may be cut in the straight section for terminating the cable. A problem occurs in that these cables must be made in extremely wide widths due to the parallel arrangement of all the untwisted wires in the straight section. Another disadvantage of a laminated twisted pair of cables is that the flat sections are not a controlled impedance section and spurious noises are introduced from one signal pair to the other signal pair.
Accordingly, an important object of the present invention is to provide a balanced transmission line cable having an accurately controlled impedance.
Another important object of the present invention is to provide a balanced line transmission cable which is more reliable and more compact than previous balanced line transmission pairs which have consisted of twisted pairs in either a laminated or woven construction.
Another important object of the present invention is to provide a balanced transmission line which has a lower level of radio frequency emissions than conventional twisted pair cables.
Yet another important object of the present invention is to provide a balanced transmission cable in which the signals are protected from external ground planes.